Interleukin-4 (IL-4) is a protein which affects a broad spectrum of hematopoietic cells [Strober et al., Pediatr. Res. 24:549 (1988)]. IL-4 enhances a number of activities including macrophage function, IgG4 and IgE production, and the proliferation of immunoglobulin-stimulated B cells, antigen-stimulated T cells and erythropoietin-stimulated red blood cell progenitors. It also increases the proliferation of IL-3-stimulated mast cells.
Together with IgE, mast cells play a central role in allergic reactions. Mast cells are granule-containing connective tissue cells which are located proximally to capillaries throughout the body, with especially high concentrations in the lungs, skin and gastrointestinal and genitourinary tracts. Following exposure to an antigenic substance, mast cells degranulate and release chemical mediators such as histamine, serotonin, heparin, prostaglandins etc. to produce an allergic reaction.
The Fc.sub..epsilon. receptor II (Fc.sub..epsilon. RII) functions in B cell differentiation and in IgE-mediated immunity. It is the low affinity receptor (10.sup.7 -10.sup.8 /M) for the Fc portion of IgE and is positioned with its amino terminus in the cytoplasm and its carboxyl terminus outside the cell [Kikutani et al., Cell 47:657 (1986)]. Fc.sub..epsilon. RII, also known as CD23 antigen, is a B cell-specific differentiation antigen restricted to mature B cells expressing IgM/IgD. Fc.sub..epsilon. RII is not found on immature bone marrow B cells, suggesting that it might be involved in the regulation of growth or differentiation of B cells.
An important role for Fc.sub..epsilon. RII in allergic reactions and immunity to parasitic infection has also been suggested, because it is present on certain populations of eosinophils and monocytes. Furthermore, interleukin-4 (IL-4), which is known to be responsible for the isotype switching of B cells to IgE, has been shown to induce Fc.sub..epsilon. RII expression on B cells [Defrance et al., J. Exp. Med. 165:1459 (1987)], monocytes [Vercelli et al., J. Exp. Med. 167:1406 (1988)], and Burkitt's lymphoma cell lines [Rousset et al., J. Immunol. 140:2625 (1988)]. The biological significance of CD23 induction on human B cells by IL-4 remains to be determined, but it has been indicated that truncated forms of CD23 can be secreted and can act as an IgE binding factor, which might be involved in the IgE-mediated immunity.
The induction of CD23 surface expression on human B and Burkitt's lymphoma (BL) cell lines by IL-4 is correlated with enhanced transcription of the specific mRNA (Rousset et al., supra). The specific promoter regulatory element for the IL-4 induction of CD23 expression in Jijoye cells (a BL cell line) has been defined by transiently expressing fusion genes with different portions of the CD23 promoter linked to a chloramphenicol acetyl transferase (CAT) reporter gene [Suter et al., J. Immunol. 143:3087 (1989)]. The genomic DNA element responsible for IL-4 induction of CD23 expression was located within the first 250 bp 5' of the transcription initiation start site.
Human IL-4 induction of this DNA element of CD23 linked to a CAT reporter gene in the transient expression system was about 2 fold. To date, this transient study appears to be the only one using a reporter gene for evaluating CD23 regulation by human IL-4. Most of the studies regarding CD23 expression have used indirect immunofluorescence staining of the Fc.sub..epsilon. RII protein.
The effector function of antibody molecules is determined by the constant region of the immunoglobulin (Ig) heavy chain (C.sub.H). Antibodies retain their specificity while their effector functions are changed by isotype switching at the DNA level.
In vitro studies using murine B cell lines indicate that Ig class switching is preceded by expression of the corresponding germline C.sub.H gene [Stavnezer et al., Proc. Natl. Acad. Sci. USA 85:7704 (1988)]. In in vitro studies of human B cells also, it has been shown that germline epsilon (.epsilon.) transcript synthesis precedes and is required for subsequent .epsilon. switching and IgE production [Gauchat et al., J. Exp. Med. 172:463 (1990)].
Rothman et al. [Mol. Cell. Biol. 11:5551 (1991)] have shown that induction of germline .epsilon. sequence transcription in an Abelson murine leukemia virus-transformed pre-B cell line is under the control of an IL-4-responsive element located at the promoter of germline .epsilon. transcripts. IL-4 is one of only two cytokines that are presently known to specifically induce germline .epsilon. sequence transcription.
Recent studies have shown that human germline .epsilon. RNA comprises, in addition to the C.epsilon. exons [Qiu et al., Eur. J. Immunol. 20:2191 (1990)], a germline .epsilon. exon located 3.5 kilobases upstream from C.epsilon. [Gauchat et al., supra; Jabara et al., J. Immunol. 145:3468 (1990)] and 5' from S.epsilon.. Synthesis of this RNA in highly purified normal B cells (Gauchat et al., supra; Jabara et al., supra) and in EBV transformed human B cells (Jabara et al., supra) or Burkitt's lymphoma cells can be induced by IL-4.
Because of the stimulatory effects of IL-4 on IgE production and mast cell proliferation, antagonists of IL-4 may be useful for the treatment of allergies by decreasing mast cell growth and IgE production. Increasing evidence suggests, however, that IL-4 may also have beneficial therapeutic applications [see, e.g., Tepper et al., Cell 57:503 (1989)]. There is thus a need to identify both antagonists and agonists of IL-4.
The search for such agonists and antagonists would be facilitated by the availability of fast and effective in vitro screening systems.